What is the best thing to do in such a situation?
Try to swim to higher ground? You can't wade it out or float on something, it seems.
If you can float, then float.
If you can grab floatation, grab floatation.
If you have any clothes on, close them up and blow air into them; when they're wet, they will hold air for some time; every lb of lift counts toward increasing your time to exhaustion.
Take off your slacks. Tie the ends of the legs together. Grab the waist and throw the opening over your head, to trap air and fill the legs with air. Clasp the waist area snug and put your head, an arm, and shoulder through the opening created by tying the legs together --- you have affected a life-ring. You'd be surprised how long you can stay afloat with that life-ring.
If you go near any water by yourself (there's nobody around to help you, who can help you), take floatation and wear it.
On any boat, on any ship, if you're out on deck when the weather is anything but tranquil, wear a life vest of some type.
No matter how unbeautiful people that may appear.
When you're young and in endurance shape, you can take chances, but little kids can't, and older people can't.